I've recently started playing the turn-based strategy game Into the Breach. I have read online that the game is considered to have 'perfect information'. What is meant by 'perfect information' in this context? In the context of Into the Breach, "perfect information" means that all information about the game state, including all future possible moves and their consequences, is available to all players at all times. This means that players have full knowledge of the game state, including the positions of all units and obstacles, and can make informed decisions based on that information. Does this mean that Into the Breach is a game of perfect information?
Yes, even though there is a lot of strategy involved all information is available to all players at all times. Chess is another example of a perfect information game. Compare this to a game like Starcraft or League of Legends. In Starcraft each player has hidden information, in League each team does.
Depends.

Does the game feature a line of sight mechanic? Are the enemies able to conceal information, such as laying down hidden traps? These would disqualify it from being a Perfect Information game.

Does the game feature randomly generated damage? If so, is the probability distribution known to the player? Can the player see the results of the PRNG roll before executing a turn? This might also disqualify it, although some academics consider chance games with known probability distributions to be valid for analysis as Perfect Information games.

Are turns executed simultaneously? If so, information required to formally reason about the current turn is hidden.